Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.147.
Section : Id prayers (Salatul ‘Idain).
The prayers of the two ‘ids was prescribed in the first year after the migration. It is a sunnah mu’kkadah as the Prophet sallallahu alehi wasallam always performed these prayers and he ordered the men and women to go out to attend them.